The value of Magnetic Resonance Imaging (MRI) devices for medical use was recognized almost immediately after they first appeared in the 1970s. Because they appear both to do no harm to the human body, and to create better images of the body's interior than the best X-ray technology, they have gained widespread use for diagnosis, pre-operative examination and even for assistance during surgical procedures. While MRI provides information on size and location of pathological abnormalities such as tumors, a variation of magnetic resonance technology called Magnetic Resonance Spectroscopy (MRS)—which identifies various biochemicals and their concentrations—often can help further by providing more information on the tissue chemistry of the target abnormality.
The clinical use of MRI/MRS (Magnetic Resonance Imaging/Magnetic Resonance Spectroscopy) technology is based on detecting and interpreting radio frequency RF (Radio Frequency) electromagnetic excitation originating from target atomic nuclei in human tissue in response to manipulation of those nuclei with magnetic fields.
In the most common MRI configurations, a static main magnetic field is produced by a solenoid magnet apparatus, and arranged so that the cylindrical space bounded by the solenoid windings (i.e. the main magnet bore) forms a convenient space and platform for placement of an object—such as the human body—containing the target nuclei. The windings of the main field are super-cooled with liquid helium in order to reduce resistance, and, therefore, to minimize the amount of heat generated and the amount of power necessary to create and maintain the main field.
Gradient magnetic fields—typically three—make it possible to both enhance and manipulate the main field at locations determined by the purpose of the medical examination. Typically, the gradient fields are created by current passing through coiled saddle or solenoid windings, which are affixed to cylinders concentric with and fitted within a larger cylinder containing the windings of the main magnetic field.
Unlike the main magnetic field, coils used to create the gradient fields are not super-cooled, and, as a result, sometimes generate enough heat to create significant discomfort for the patient being examined. Any effort to reduce patient discomfort by placing cooling units within the walls of the cylinder between the patient and the gradient coils increases the distance between the patient and the coils that create the gradient fields, which reduces the efficiency of the gradient coil. A greater distance means a gradient coil will have higher resistance and inductance for a given coil gain (gradient strength per unit current) and for a given gradient field linearity, which results in higher heat generation and lower coil switching speed. If the coil gain or the linearity is reduced to maintain low resistance and inductance, the total gradient strength for a given current will be lower or the gradient field linearity will be poorer. The gradient strength and field linearity are of fundamental importance both to the accuracy of the details of the image produced and the information on tissue chemistry.
Increasing the current in the coils to improve the gradient strength at the greater distance not only increases power consumption raising the cost of operation of the MRI/MRS device, but defeats the purpose of the cooling units by increasing the heat generated by the coil.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for decreasing the heat experienced by the patient without compromising the clinical value of the gradient fields.
The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification.